The present invention relates to electric resistance heating elements. More particularly, the invention relates to an insulating standoff and support structure for a helical wire heating coil used in such heating elements.
Electric heating elements utilizing helical wire heating coils are old and well known in the art. A helical wire heating coil is typically mounted on a supporting structure and strung between a number of ceramic insulating standoffs that provide direct support for the heating coil and isolate the heating coil from the supporting structure, which is generally some type of metal framework. It is important that the insulating standoffs hold the coil against both lateral displacement out of the individual standoff and movement in the direction of the longitudinal axis of the coil. Thus, it is common in the prior art ceramic insulating standoffs to capture one or more turns of the helical coil to hold the same against lateral displacement and axial movement.
One common prior art standoff is typified by the constructions shown in U.S. Pat. Nos. 4,363,959 and 4,692,599. In each of these patents, a ceramic insulating standoff for the helical coil of a heating element includes a generally thin, flat body with two or more hook-like notches on one or both ends. A few turns or convolutions of the heating coil are separated slightly and retained in the hook-like notches by the inherent resiliency of the coil. The longitudinal axis of the coil extends generally parallel to the thin, flat body of the insulator with adjacent turns of the coil held in oppositely facing notches. To attach the coil to the supports, the coil must be stretched axially and/or twisted rather severely from its axial direction, resulting in the possibility of stretching the wire beyond its yield point and causing a permanent deformation to the coil.
Another somewhat similar insulating standoff is shown in U.S. Pat. No. 4,250,399. The insulator shown in this patent also has a relatively thin, flat ceramic body with a single coil supporting notch centered in one edge. The notch extends generally perpendicular to the flat body and supports a portion of the coil. The edge of the insulator body on both sides of the notch is provided with downwardly opening lips which engage the coil turns on each face of the body to prevent the coil from being withdrawn after attachment. In order to attach the coil to the insulator body, however, the coil must be turned so that the coil axis is 90.degree. to its final position in order to insert one turn of the coil into the slot. Additionally, the insulator is connected to a metal framework by a finger formed on the framework that is received in an opening in the insulator. The assembly shown in the '399 patent requires a complicated procedure for both mounting the insulating standoffs to the support frame and for mounting the coil to the insulating standoffs, which can be tedious, time-consuming and costly.
U.S. Pat. Nos. 4,472,624, 4,528,441, and 4,628,189 all disclose somewhat similar insulating standoffs that attempt to solve certain of the assembly problems described above. Each of these patents utilizes a construction intended to obviate the need to twist and distort the coil before its attachment to the standoff. However, each of the insulators in the foregoing patents engages and supports three consecutive convolutions of the coil, in some cases requires distortion of the coil beyond a mere spreading of the convolutions, and all have rather narrow bodies in the direction transverse to the coil axis which do little or nothing to prevent lateral movement of the coil after attachment to the insulator.
Above identified U.S. Pat. No. 4,692,599 utilizes a supporting frame for the insulating standoffs comprising circular section wire rods which are wrapped by multiple bending operations around the insulator bodies to hold them in place. The process of preforming, bending and closing the wire rods around the insulating bodies is complex and time consuming.
A number of the patents identified above utilize stamped sheet metal frames or bars to support the insulating standoffs. In Pat. Nos. 4,472,624 and 4,628,189, the insulators are pushed through slots in the stamped supporting frame and turned 90.degree. allowing edges of the slots to be captured in grooves in the insulator body. In U.S. Pat. Nos. 4,250,399 and 4,528,441, tabs on the stamped sheet metal frame member are inserted into or through apertures in the insulator body and twisted or bent to retain the insulator in position.
All of the foregoing methods and apparatus for supporting the insulating standoffs are difficult or virtually impossible to automate, thereby requiring substantial manual labor in the assembly process.
In addition to the insulating standoffs shown in the previously identified U.S. patents, U.S. Pat. No. 5,122,640, commonly owned by the assignee of the present application, discloses another heating element coil support. The insulating support shown in the '640 patent includes a plurality of rectangular insulating supports, each of which retains and supports four separate coil portions. Although the insulating support shown in the '640 patent functions to retain the heating coil as desired, the relatively large ceramic insulating supports are relatively heavy and expensive to manufacture.
It would be most desirable to have an insulating standoff and support structure for a helical wire heating coil in which the coil is retained against either axial or lateral movement and the insulating standoffs can be easily attached to the support structure. It is also desirable to have an insulating standoff and support structure that lend themselves to fully automated assembly. Similarly, an insulating standoff constructed to permit direct linear insertion into the heating coil without undue coil distortion would also facilitate automated assembly of the coil to the standoffs.